GB2069704A - Blood pressure measuring equipment - Google Patents

Abstract

Blood pressure measuring equipment comprises a sleeve (1) with an inflatable air chamber (31) and a microphone (33), and an appliance (3) for measurement and indication of systolic and diastolic blood blood pressure. Sound signals delivered by the microphone (33) during measurement are fed through filter means (51) to a discriminator (53) which generates a pulse for each Korotkoff sound. Diastolic pressure is measured on the last pulse that is formed with the air chamber decreasing. The transfer function of the filter means (51) can be changed in such a manner that diastolic pressure is measured selectably at the so-called fourth or fifth Korotkoff sound eg. by using a transfer function characterised by an enhanced lower frequency transmission to detect the softer fifth Korotkoff sound. In this manner, the blood pressure measurement can be adapted to the individual characteristics of a patient. <IMAGE>

Description

SPECIFICATION Blood pressure measuring equipment The present invention relates to blood pressure measuring equipment.

Blood pressure measuring equipment described in US patent specification No. 2 827 040 comprises a microphone for detection of the Korotkoff tones generated by blood flowing through an artery. The microphone is connected via an amplifier, a bandpass filter and a pulse shaper with a coincidence circuit. An inflatable sleeve, which is attachable to the arm of the person to be examined, is connected with an air reservoir having an outlet nozzle which, during measurement, generates an air jet flowing past a thermistor. The thermistor, which serves for detection of pressure pulses, is similarly connected via an amplifier and a pulse shaper with the coincidence circuit. In addition, a manometer for detection of systolic pressure and a manometer for detection of diastolic pressure are provided. Each of the manometers is connected by way of respective valve with the air reservoir.The air reservoir is connected by way of a valve with a compressor and additionally via a venting valve with the ambient atmosphere. Control equipment for actuation of the different valves is also included.

During blood pressure measurement, the pressure in the air reservoir is progressively increased. Pulses are then generated in certain pressure ranges by Korotkoff tones and by pressure fluctuations and these pulses are fed to the coincidence circuit. On the first instance of coincidence of pulses from the two sources, i.e. the coincidence occurring at the lowest pressure, the manometer for measurement of diastolic pressure is temporarily connected with the air reservoir so that it measures and indicates the diastolic pressure. Thereafter, the pressure is further increased. When the pulse coincidence at the highest pressure occurs, the manometer serving for measurement of systolic pressure is temporarily connected with the air reservoir and the systolic pressure is then measured.

This prior art equipment has the disadvantage that its appliance part connected to the sleeve must include an air reservoir and four valves, which makes it correspondingly large and awkward and difficult to use as a manual appliance for singlehanded operation by a physician or other person. A further disadvantage is that detection of pressure fluctuations by means of a thermistor cooled by an air current is both delicate and subject to inaccuracies.

Moreover, the temporal course of measurement in this automatic appliance is significantly different from the measurement procedure usually followed by physicians when determining blood pressure by conventional manual appliances. With these appliances, a measuring sleeve is initially rapidly pumped up to a pressure above systolic pressure.

Thereafter, the sleeve is slowly vented while the onset and conclusion of the production of Korotkoff tones is determined with the aid of a stethoscope.

The duration of the pressurisation of the sleeve and the temporal course of the pressure can, however, influence the measurement results. An automatic appliance in which the temporal pressure course is substantially different from the above-described traditional measurement process carried out by physicians can give rise to measurement errors or, at the least, render more difficult a comparison of measurement results obtained by the automatic appliance with those obtained by a traditional method.

Another blood pressure measuring equipment described in US patent specification No. 3 131 comprises a microphone which is connected via a regulated amplifier with the inputs of three different bandpass filters having transmission frequencies of, respectively, 40, 100 and 1000 Hertz. The outputs of the three bandpass filters are connected to a logic circuit. The equipment also includes an inflatable sleeve and a pressure sensor which is connected through an analog-digital converter, which can be switched on and off, and a gate circuit with a pressure recording appliance.

For blood pressure measurement, the sleeve is inflated to a pressure above systolic pressure and then slowly vented. Korotkoff tones are then generated in a certain pressure range and are converted by the microphone into electrical signals. The logic circuit connected to the output of the filter operates in such a manner that it can identify signals with a 1000 Hertz component as noise signals, and signals with a 40 and a 100 Hertz component, but no 1000 Hertz component, as Korotkoff tones. On each identification of a signal as a Korotkoff tone, the analog-digital converter and the gate circuit are controlled by the logic circuit in such a manner as to cause the instantaneous pressure measured by the pressure sensor to be recorded in the pressure recording appliance.The first recorded pressure value then corresponds to systolic pressure and the last recorded pressure value to diastolic pressure.

There is also the possibility of providing an additional circuit which records only the systolic and diastolic pressures.

In the case of the blood pressure measuring equipment described in US patent specification Nos.

2827040 and 3450 131,the smallest pressure at which a Korotkoff tone appears is thus measured as diastolic pressure. In that case, the presence of a Korotkoff tone is ascertained when the corresponding electrical signals, which pass the bandpass filter included in the equipment, exceed a certain threshold value.

Comparison measurements are now performed in which diastolic blood pressure values are measured on the one hand indirectly, i.e. with the use of an inflatable sleeve, and on the other hand directly, i.e.

by direct connection of the arteries with a pressure measuring device. It is evident from these comparison measurements that the indirect and direct measurement methods have different degrees of agreement for different patients, and appreciable deviations can appear. If it is considered that the arterially measured pressure is the true blood pressure, then any disconformity with the results of indirect measurement method is an indication of measurement errors in the latter method, the magnitude of these errors being dependent on the individual characteristics of the patients concerned.

It has in fact been known for some time that on reduction of the sleeve pressure to the range of the diastolic pressure the Korotkoff tones not only become softer, but also change their frequency. In this connection reference is made to, for example, "Handbuch der inneren Medizin, Volume 5, fifth part "Herz und Kreislauf" published in 1960 in Springer Verlag by G. von Bergman, W. Frey and H. Schwiegk.

In the specialised field, so-called fourth and fifth Korotkoff tones are differentiated by physicians. The fifth tone is softer and lower than the fourth tone. At present, it is recommended by standing bodies of physicians to measure diastolic blood pressure at the fourth Korotkoff tone.

Examinations have now shown that diastolic blood pressure measured at the fourth Korotkoff tone is about 7 to 15 % greater, in about 80 to 90 % of examined persons, than the pressure measured at the fifth Korotkoff tone. Such a difference is in general of little significance in view of the transient blood pressure changes that normally occur. However, with about 10 to 20 % of examined persons the diastolic blood pressure measured at the fourth Korotkoff tone is up to 70 % greater than the pressure measured at the fifth Korotkoff tone.

In the case of the afore-mentioned prior art blood pressure measuring equipment, the diastolic blood pressure is always measured at the same Korotkoff tone, for which purpose the fourth Korotkoff tone is no doubt concerned. Consequently, appreciable measurement errors can arise in a proportion of the examined persons.

There is accordingly a need for blood pressure measuring equipment for indirect blood pressure measurement in which the measurement errors as described in the foregoing can be eliminated or at least reduced. The provision of equipment achieving this result may proceed from, in particular, recognition of the fact that measurement errors appearing in indirect blood pressure measurement can be reduced by taking into account not only the change of volume but also the individually different frequency spectra of the Korotkoff tones.

According to the present invention there is provided blood pressure measuring equipment comprising tone detecting means for detecting tones generated by blod flow in an artery and providing signals indicative of detected tones, and filter means connected to the tone detecting means to receive and to filter such signals and having at least two different transfer functions which each have a signal transmission factor dependent on frequency in a manner different to that of the or each other transfer function, selecting means being provided for selecting an individual one of the transfer functions.

For clarification, it is noted that the references in the following description and claims to blood pressure and air chamber pressure are to be understood as denoting the excess pressure measured with respect to ambient air pressure.

An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a schematic view of blood pressure measuring equipment according to the said embodiment, Figure 2 is a schematic block diagram of the principal electronic and pneumatic components of the equipment of Figure 1, Figure 3 is a circuit diagram of filter means in the equipment, for selectable detection of different Korotkoff tones, Figure 4 is a diagram illustrating the frequency course of the transfer function of the filter means of Figure 3,and Figure 5 is a diagram illustrating the temporal course of a blood pressure measurement by the equipment.

Referring now to the accompanying drawings, in Figure 1 there is shown blood pressure measuring equipment comprising a sleeve 1 attachable to the arm of a person to be examined and an appliance indicated generally by 3. The sleeve 1 comprises a rubber bag defining a deformable and inflatable air chamber and contains a microphone. The sleeve 1 is detachably connected to the appliance 3 by a line 5, which comprises an air hose connected to the air chamber and a cable connected to the microphone, the line being provided at the appliance end with a coupling socket 7. The appliance 3 comprises a housing 9 provided with a threaded shank 9a to which a pump 13 with a substantially cylindrical rubber pump bellows is detachably fastened by means of a box nut 11.An air hose connection nipple 15 and an electrical connection pin 17, formed by a chassis plug, are provided on the housing 9 for coupling thereto of the socket 7. A connection element 19, also formed by a chassis plug, is included for the connection of a headphone. The appliance 3 also comprises three press key switches 21,23 and 25, a digital indicating unit 27 and various pneumatic and electronic components, as will be subsequently described, accommodated in the in teriorofthe housing 9.

Figure 2 shows the inflatable air chamber, rnfer- enced 31, and the microphone, referenced 33, of the sleeve 1 as well as some of the pneumatic and electronic components in the appliance 3. The air chamber 31 is connected by the air hose in the line 5, and by air lines in the appliance 3, via a non-return valve 35 with the pump 13, an electrically controllable outflow valve 37 and a pressure sensor 39. The pump 13 is provided with an air inlet having a non-return valve 41. The two non-return valves 35 and 41 are so arranged that by alternating manual compression and release of the pump bellows air can be sucked from the ambient atmosphere and pumped into the air chamber 31.

The microphone 33 is connected by electrical conductors with the input of a filte means 51, the output of which is connected with the headphone connection element 19 and with a discriminator 53, which comprises a trimming potentiometer 54 for setting or a lower threshold value and a pulse shaper. The output of the pulse shaper is connected to a control unit 55.

The pressure sensor 39 comprises a measurement converter bridge circuit formed by piezo-resistive elements and is connected with the input of an amplifier 57, the output of which is connected via a differentiator 59, and via a parallel connection bridging the differentiator, with the control unit 55.

The control unit 55 and the amplifier 57 are also connected at outputs thereof with a device 75 for automatic zero balancing, the output of the device 75 being connected to the pressure sensor 39. The output of the differentiator 59 is also connected to the control unit 55 and additionally to an input of a regulator 61. The control unit 55 is connected to another input of the regulator 61, the output of which is connected with an electromagnetic actuating means of the outflow valve 37. The control unit 55 additionally has two connections which are connected to, respectively, two analog stores 63 and 65 each formed by a respective capacitor, and is connected to an indicating control device 67. The device 67 includes, amongst other things, an analogdigital converter and is connected to the indicating unit 27.

A discriminator 69 is connected at its input to the output of the differentiator 59, and at its output to an input of a heartbeat frequency meter 71 and an input of the control unit 55. The meter 71 is connected at a control input thereof to an output of the control part 55 and comprises an analog store connected at an output to an input of the control unit 55. The switch 21 is connected to the control unit 55 and the switch 23 to the indicating control device 67. Also present is a voltage source 73, which includes a battery and which is connected to supply voltage connections of the different operative components and to an earth connection. The switch 25 and also the control unit 55 are connected to the voltage source 73, which, apart from the battery comprises logic elements and a regulator for stabilisation of the supply voltage.

The battery is accommodated in a battery compartment closably by a lid.

As shown in Figure 3, the microphone 33 is connected with an input connection 81 and an earth connection 83 of the filter means 51. The connection 81 is coupled through a capacitor 84 with the non-inverting input 85a, and through a resistor 87 with the inverting input 85b, of an amplifier 85. The non-inverting input 85a is connected with two parallelly connected resistors 89 and 91, the resistor 89 being connected directly, and the resistor 91 via a switch 93, with the earth connection 83. The output 85c of the amplifier is connected via a resistor 95, and a capacitor 97 connected in parallel therewith, back with the inverting input 85b, which is also connected with the earth connection 83 via a resistor 99 with a series-connected capacitor 101. The amplifier output 85c is connected with the output connection 103 of the filter means.In addition, the amplifier 85 is connected by conductors (not shown) with the supply voltage source 73.

The trimming potentiometer 54 and the switch 93 are accommodated in the interior of the housing 9 so they can be adjusted or actuated only after opening the lid which closes the battery compartment.

The ratio of the voltage measured with respect to earth at the output connection 103 and the voltage measured with respect to earth at the input connection 81 is designated as transmission factor T. Figure 4 shows the dependence of the transmission factor on the frequency f for both the settings of the switch 93. When the switch 93 is open as shown in Figure 3, the transfer function of the filter means is represented by the transmission factor according to the curve 111. When the switch 93 is closed, the transmission factor runs according to the curve 113.

The transmission factor represented by the curve 11 has a rather sharp maximum at the frequency f, this maximum being designated T2 and having a value of, for example, about 100. The lower limit frequency, at which the transmission factor according to the curve 111 is equal to T21 < , is designated by f1. The curve 113 rises slowly, reaches a plateau-shaped maximum and then again reduces, the maximum transmission factorof the curve 113 being designated T1.The lower limit frequency of the transfer function represented by the curve 113 is designated by f3. The plateau-shaped and falling part of the curve 113 at least approximately coincides with parts ofthecurve 11.Ascan be seen from Figure 4, the lower limit frequency f3 is higher than the limit frequency fa and the frequency f2 as a result of the course represented by the curve 113. The lower limit frequency fl of the frequency course according to the curve 111 is at least 10 Hertz and at most 30, preferably at most 25, thus, for for example about 20 Hertz. Conversely, the limit frequency f3 is at least 35, preferably at least 40, and at most about 60 Hertz, thus for example about 50 Hertz.The frequency f2, at which the curve 111 reaches its maximum, is about 25 to 35, for example 30, Hertz. The plateau-shaped maximum of the curve 113 is disposed above f2 and extends from about 80 to 120 Hertz. The upper limit frequency for both curves lies at about 130 to 200 Hertz. The maximum transmission factor T2 of the curve 11 is at least 30 %, for example about 50 %, greater than the maximum transmission factor T1 of the curve 113.

It is to be noted that the electronic components illustrated in Figures 2 and 3 can be assembled into integrated switching circuits.

The operation of the blood pressure measuring equipment will now be explained in detail with reference to Figure 5.

For performance of a measurement, the sleeve 1 is connected by the line 5 with the appliance 3 and is attached to the arm of the person to be examined.

The dimensions of the appliance 3 are such that it can conveniently be held by one hand, for which purpose the pump 13 also serves as a handgrip.

When necessary, all three press key switches 21,23 and 25 can be actuated by the hand holding the appliance.

To begin with, the change in pressure p in the air chamber 31 in relation to the course of time twill be discussed. The temporal course of the pressure p is represented by the curve 121 of the diagram illustrated in Figure 5. When the sleeve is attached, the appliance is made operationally ready at the instant to through a brief depression of the ONIOFF switch 25. In the time interval from the instant to to the instant t1, the pressure sensor 39 is automatically balanced to zero by the zero balance device 75. The end of this balance at the instant t1 is signalled by the indicating unit 27 indicating the value zero.

Between the instants t, and t2, air is pumped intermittently into the air chamber 31 by the pump 13 so that an air pressure arises in the chamber which is greater than the systolic pressure. Shortly after the termination of the inflation process, in particular at the instant fs air starts to flow out of the air chamber 31 through the valve 37 and into the environment so that the pressure in the air chamber falls. In that case, a voltage proportional to the pressure p is generated by the pressure sensor 39 and the differential quotient dp!dt is determined by the differentiator 59.The regulator 61 regulates the outflow valve 37 in such a manner that the differential quotient dpldt remains constant apart from pressure fluctuations, which are caused by heart activity and which will be further explained, during the actual measurement phase.

When the pressure p is now reduced from its maximum pressure lying above the systolic pressure ps, the pressure fluctuations caused by heartbeats occur from the instant 4. These pressure fluctuations are detected by the differentiator 59. The discriminator 69 then generates a pulse on each pressure fluctuation generated bya heartbeatfor which the differential quotient dpldt exceeds a predetermined threshold value of at least 100 Pascals per second, for example 400 Pascals per second.

This pulse sequence is designated by 123 in the Figure 5.

When the pressure in the air chamber 31 is reduced to be in a certain range, blood flowing through the artery enclosed by the sleeve 1 generates noises, the so-called Korotkoff tones, on each blood stroke generated by a heartbeat. These Korotkoff tones are converted by the microphone 33 into electrical signals and transmitted through the filter means 51, which preferably also amplifies the signals to the discriminator 53. When the voltages of the Korotkoff tone signals exceed the lower threshold value determined by the discriminator 53, the pulse shaper of the discriminator feeds a respective pulse to the control unit 55. This pulse sequence is designated by 125 in Figure 5 and extends from the instant t5 to the instant t6.

In the control unit, the pulses generated through the pressure fluctuations and the pulses generated through the Korotkoff tones are fed to an AND-gate.

As is evident from Figure 5, the pulses generated through the pressure fluctuations are wider than the pulses generated through the Korotkoff tones. The AND-gate forms a coincidence circuit and operates a window for the pulses of the pulse sequence 125 during each pulse of the pulse sequence 123. Signals from the microphone are thus further processed only when they fall into a window opened by a pressure fluctuation, i.e. when a coincidence exists between the tone signals and the pressure fluctuations. As a result, the Korotkoff tones can be distinguished from interfering noises and the latter be suppressed.

The control unit 55 includes an electronic switching device which connects the output of the amplifier 57 with the store 63 when the appliance is switched on.

The control unit 55 also includes means to ascertain the appearance of the first Korotkoff signal passing the afore-mentioned AND-gate. When the first Korotkoff signal arrives, the line from the amplifier 57 of the pressure-measuring channel is separated from the store 63. The store 63 accordingly stores the pressure value present on the arrival of the first Korotkoff tone signal, i.e. the systolic pressure.

As the pressure in the air chamber 31 drops, further Korotkoff tones follow the first Korotkoff tone. The control unit 55 comprises means which briefly connect the output of the amplifier 57 with the store 65 on each Korotkoff tone, i.e. on each pulse of the pulse sequence 125. A new pressure value is thus stored in the store 65 on each Korotkoff tone, these pressure values progressively reducing. As already mentioned, the pulse sequence 125 extends to the instant t6. As no further pulses occur after the instant t6, the value of the pressure p measured at the instant t6 remains stored in the store 65 until the appliance is switched off. This storage value then represents the diastolic pressure.

The control unit 55 also comprises circuit means by which it can be ascertained when no further Korotkoff tone has occurred during a predetermined time interval of 2 to 10, for example 5, seconds. At the end of this time interval, namely at the instant7, the control unit 55 delivers to the regulator 61 a signal which has the effect that the valve 37 is fully opened. The pressure p then drops very rapidly and at the instant to is again at the value zero, i.e. the ambient air pressure.

The control unit 55 is so constructed that it connects the output of the amplifier 57 with the indicating control device 67 at regular time intervals of, for example, 0.3 seconds up to the instant t7. The indicating unit 27 then indicates the instantaneous.

pressure each time. The control unit 55 could, however, also be constructed in such a manner that the pressure would be indicated on each pulse of the pulse sequence 123 in the time interval between the instants t4 and t7.

The control unit also switches the heartbeat frequency meter 71 on temporarily so that this measured the heartbeat frequency during the occurrence of the pulse sequence 123 and determines a mean value. This is stored in the store of the meter 71.

The nature of the control unit 55 is also that the store 63, the store 65 or the store of the meter 71 can be cyclically interrogated by a brief depression of the switch 21. The relevant storage value stored in analog form is then fed to the indicating control device 67 and converted by this into a digital signal.

This is fed to the indicating unit 27 so that the unit thus selectably indicates systolic or diastolic pressure or heartbeat frequency. The indicating control device 67 includes a network which can be switched over by the switch 23 and connected between the feed lines of the stores 63 and 65 and the analogdigital converters. This makes possible selectable presentation of the pressure indication in kilopascals or torrs, the switching between these alternative forms of indication being effected by a brief depression of the switch 23.

When all three storage values have been read off, the appliance 3 can be switched off by depressing the ON/OFF switch 25, whereby the measurement is concluded.

Now that the general mode of operation of the equipment has been explained, the purpose of the switching capability of the transfer function of the filter means will be explained in more detail.

As will be apparent from the preceding explanation, the instantaneous pressure present in the air chamber 31 during the last pulse of the pulse sequence 125 is identified and measured as diastolic pressure PD. When the pressure reduces and approaches the diastolic pressure, the Korotkoff tones become softer. In addition, their frequency spectra is displaced towards lower frequencies.

Such a displacement to low frequencies takes place, in particular, between the tones designated as fourth and fifth Korotkoff tone. These two tones are identified by physicians on the basis of their relative volume and their different sound. As analyses of the frequency spectra have shown, the loud components of the frequency spectrum for the fourth Korotkoff tone by and large lie above 50 to 80 Hertz.

Thereagainst, the fifth Korotkoff tone has a volume maximum in the neighbourhood of 30 Hertz.

When the switch 93 is open and the transfer function of the filter means 51 is represented by the curve 111 of Figure 4, the frequencies in the region of the frequency ~2, which as mentioned is, for example, about 30 Hertz, are raised. In this case, the electrical tone frequency signal generated by the fifth Korotkoff tone and fed to the discriminator 3 is raised to such an extent that it exceeds the threshold value of the discriminator 53 and effects the generation of one pulse of the pulse sequence 125. The diastolic pressure is thus measured on the fifth Korotkoff tone.

When the switch 93 is closed and the transfer function of the filter means 51 is represented by the curve 113, the transmission factor T has only a relatively low value in the neighbourhood of the frequency f2. The electrical signal generated by the fifth Korotkoff tone and fed to the discriminator 53 is then no longer sufficient to exceed the threshold value of the discriminator 53. In that case, the last pulse of the pulse sequence 125 is generated by the fourth Korotkoff tone so that the diastolic pressure is measured on the fourth Korotkoff tone.

It is again pointed out that the fourth and fifth Korotkoff tones are identified by physicians on the basis of their relative volume and their sound. The fourth and fifth Korotkoff tones are therefore not necessarily identical with the fourth and fifth pulses, respectively, of the pulse sequence 125.

The blood pressure measuring equipment hereinbefore described is suitable for, inter alia, patients who measure their blood pressure themselves and from time to time place their measurement results before the physician treating them. As mentioned in the introduction, individual characteristics of the patients, particularly different artery formations, can influence the volumes and frequency spectra of the Korotkoff tones. However, the physician can now perform blood pressure measuring with the equipment during an examination of the patient. The physician has the option of listening to the Korotkoff tones with a stethoscope or with a headphone connected to the connection 19 and to continuously read off the instantaneous pressure.

The physician can then draw on his acoustic judgement of the Korotkoff tones and his other knowledge of the patient to decide whether it would be sensible in a given case to measure the blood pressure at the fourth or at the fifth Korotkoff tone. If the volume of the Korotkoff tones differs strongly from the usual value, the physician can, by adjusting the trimming potentiometer 54, adapt the threshold value of the discriminator 53 to the individual characteristics of the patient. Through selection of the fourth or fifth Korotkoff tone as a criterion for measurement of diastolic pressure and, if so desired, through variation of the discriminator threshold value, the degree of agreement between diastolic pressure values measured indirectly by equipment and directly via an artery can be improved.Moreover, the significance of the measurements is improved and is more readily comparable with the measurement values of other patients.

The equipment can be modified in a number of different ways. For example, the filter means connected between the microphone and the discriminator could be constructed in such a manner that not only two, but selectably three or more different transfer functions can be switched. As a result, the transfer function could be even more closely adapted to the individual characteristics of a particular patient. In addition, the selection of the transfer function could be carried out not through a switch, but through another manually settable device, for example, a potentiometer so that a continuous influencing of the frequency course of the transfer function would then be possible.

It is also pointed out that, instead of a separate microphone and a separate pressure sensor, a combined sound-pressure pick-up could be provided, the pick-up serving for the detection of the tones generated by the blood as well as also for the detection of the quasi-static blood pressure and the pressure modulation produced by the heartbeats.

The pick-up could be arranged either in the inflatable sleeve or in the appliance. Electrical signals delivered by the pick-up could then be split up by a frequency band dividing filter and fed, as appropriate, to the sound channel and the pressure channel of the electronic system.

Finally, reference is made to the specifications of Swiss patent application Nos. 1296/80,1298/80 and 1299/80 of the applicant in which further details of such blood pressure measuring equipment are described.

Claims (9)

1. Blood pressure measuring equipment comprising tone detecting means for detecting tones generated by blood flow in an artery and providing signals indicative of detected tones, and filter means connected to the tone detecting means to receive and to filter such signals and having at least two different transfer functions which each have a signal transmission factor dependent on frequency in a manner different to that of the or each other transfer function, selecting means being provided for selecting an individual one of the transfer functions.

2. Equipment as claimed in claim 1, the selecting means comprising a switch.

3. Equipment as claimed in either claim 1 or claim 2, wherein the transfer functions have respectively different lower limit frequencies.

4. Equipment as claimed in claim 3, wherein the lower limit frequency of one of said two transfer functions is 10 to 30 Hertz and the lower limit frequency of the other one of said two transfer function is at least 35 Hertz.

5. Equipment as claimed in any one of the preceding claims, wherein said two transfer functions have respectively different maximum transmission factors.

6. Equipment as claimed in claim 5 when appended to either claim 2 or claim 3, wherein the maximum transmission factor of that one of said two transfer functions with the smaller lower limit fre quency is greater than that of the other one of said two transfer functions.

7. Equipment as claimed in any one of the preceding claims, further comprising measurement means attachable to a person and provided with a chamber inflatable by fluid pressure, a pressure sensor responsive to fluid pressure in the chamber to provide a signal indicative of the pressure, differentiating means connected to the sensor to provide a signal indicative of the rate of change in the pressure, a first discriminator connected to the differentiating means to receive signals therefrom and to provide an output signal whenever the magnitude of a received signal exceeds a first predetermined threshold value, a second discriminator connected to the filter means to receive signals therefrom and to provide an output signal whenever the magnitude of a received signal exceeds a second predetermined threshold value, and control means connected to the discriminators to receive the output signals thereof and adapted to determine simultaneous occurrence of the output signals of the first discriminator with those of the second discriminator thereby to determine simultaneous occurrence of the detected tones and heartbeat-induced fluctuations in the fluid pressure.

8. Equipment as claimed in claim 7, further comprising fluid outflow means for controlling outflow of fluid from the chamber thereby to reduce the fluid pressure therein, and store means for storage of measurement values of the pressure, the store means being controllable by the control means to store the lowest value of the pressure at which the detected tones and the heart-beat-induced pressure fluctuations occur simultaneously.

9. Blood pressure measuring equipment substantially as hereinbefore described with reference to the accompanying drawings.